Electrosurgical generator and system

ABSTRACT

An electrosurgical system including an electrode assembly having two electrodes for use immersed in an electrically conductive fluid has a generator with control circuitry for rapidly reducing the delivered radio frequency output power by at least 50% within at most a few cycles of the peak radio frequency output voltage reaching a predetermined threshold limit. In this way, tissue coagulation can be performed in, for example, saline without significant steam generation. The same peak voltage limitation technique is used in a tissue vaporization or cutting mode to limit the size of the steam pocket at the electrodes and to avoid electrode burning. In a blended mode, the output voltage is alternately limited to a value appropriate for coagulation and a value appropriate for cutting or vaporization.

RELATED APPLICATION

[0001] The present application is a continuation-in-part of applicationSer. No. 08/642,121, filed May. 2, 1996.

FIELD OF THE INVENTION

[0002] This invention relates to an electrosurgical generator fordelivering an electrosurgical current particularly but not exclusivelyin intracavitary endoscopic electrosurgery. The invention also relatesto an electrosurgical system comprising the combination of a generatorand an electrode assembly. The term “intracavitary” is used in thisspecification to denote electrosurgery in which living tissue is treatedby least invasive surgical access to a body cavity. This may involve“underwater electrosurgery”, a term denoting that the surgery isperformed using an electrosurgical instrument with a treatment electrodeor electrodes immersed in liquid at the operation site. The inventionhas particular application in the fields of urology, hysteroscopy andarthroscopy.

BACKGROUND OF THE INVENTION

[0003] Intracavitary endoscopie electrosurgery is useful for treatingtissue in anatomical or surgically created cavities of the body whichcan be accessed by methods involving minimal trauma to the patient, bethis through a natural body passage or one created artificially. Thecavity is distended to provide space for gaining access to the operationsite to improve visualisation and to allow for manipulation ofinstruments. In low volume body cavities, particularly where it isdesirable to distend the cavity under higher pressure, liquid ratherthan gas is more commonly used due to better optical characteristics andbecause it washes blood away from the operative site. Conventionally. anon-electrolyte solution such as glycine is used as the fluid distensionmedium when electrosurgery is being used, glycine being electricallynon-conductive.

[0004] The limited surgical access encountered during intracavitaryendoscopic procedures makes the removal of tissue pieces derived from atypical electrosurgical loop cutting electrode both difficult and timeconsuming. Vaporisation of tissue whereby the tissue is reduced to smokeand water vapour is a preferable technique in these situations, ratherthan the piecemeal removal of tissue. The products of vaporisation maybe removed following dissolution within a liquid irrigating medium.

[0005] With regard to underwater endoscopic electrosurgery, theapplicants have found that it is possible to use a conductive liquidmedium such as normal saline in place of glycine. Normal saline is thepreferred distension medium in underwater endoscopic surgery whenelectrosurgery is not contemplated or a non-electrical tissue effectsuch as laser treatment is being used. Although normal saline (0.9% w/v;150 mmol/l) has an electrical conductivity somewhat greater than that ofmost body tissue, it has the advantage that displacement by absorptionor extravasation from the operative site produces little physiologicaleffect and the so-called water intoxication effects of glycine areavoided.

[0006] Effective electrosurgical treatment of tissue which is totallyimmersed in liquid at the application site is difficult to achievebecause the heat generated by the flow of electrical currents in boththe tissue being treated and surrounding conductive liquid tends tocause boiling of the liquid. The operating electrode is intermittentlysurrounded by water vapour rather than liquid, with consequent largevariations in the electrical impedance of the load presented to thegenerator supplying the electrosurgical power to the electrode. Whilstthis variation is mitigated by use of a non-conductive liquid, it cannotbe eliminated entirely due to the release of body fluids at theoperative site which elevates the electrical conductance of the liquid.Changes in tissue type also alter the load impedance. These effectsresult in difficulty in controlling the electrosurgical output toproduce consistent effects on the tissue being treated. As a result,high powers are commonly employed to overcome this performancevariation.

SUMMARY OF THE INVENTION

[0007] According to a first aspect of this invention, an electrosurgicalgenerator for supplying radio frequency power to an electricalinstrument, comprises a radio frequency output stage having at least apair of electrosurgical output connections for the delivery of radiofrequency power to the instrument, and control circuitry operable tolimit the radio frequency peak output voltage developed across theoutput connections to at least first and second predetermined thresholdvalues and, in a blend mode of the generator, to alternate constantlybetween said first and second threshold values. The output stagepreferably comprises a resonant output circuit coupled to the outputconnections and a switching device coupled to the resonant outputcircuit, and wherein the control circuitry is operable to actuate theswitching device to reduce the delivered radio frequency power. Theswitching device is preferably connected between the resonant outputcircuit and one of a pair of supply rails of the power supply means, andconnected so as to switch current repeatedly through the resonant outputcircuit at its resonant frequency. In order to cause a controlovershoot, in terms of the degree to which the delivered power isreduced when the output voltage reaches the predetermined threshold, thecontrol circuitry is so arranged and coupled to the switching devicethat it is capable of reducing the “on” time of the switching deviceduring individual radio frequency switching cycles sufficiently rapidlyto cause a 50% reduction in delivered output power within 100 μs of thepredetermined threshold having been reached. This allows surgery to beperformed in a conductive fluid field, in particular in a salinesolution. Large and rapid changes in load impedance can occursubstantially without causing unwanted electrosurgical effects. Forexample, when it is desired to produce electrosurgical desiccation, anyincrease in impedance due to vaporisation of surrounding saline in theregion of an electrode of the instrument which might otherwise lead tounwanted arcing at the required power level for effective desiccationcan be largely prevented. When electrosurgical tissue cutting or tissuevaporisation is required, output voltage limitation can be used toprevent electrode burning and/or excessive tissue vaporisation. In theblended mode, the above two states are used alternately, wherein apocket of vapour continually forms and collapses in rapid succession.

[0008] The control circuitry may include a control line feeding a firstpower reduction control signal to the radio frequency output stage. Theoutput stage, which may be a radio frequency power oscillator, typicallyhas as the oscillating element a radio frequency power device. and inthe preferred embodiment, the control circuitry is arranged such that atleast a 50% reduction in output power is brought about in a period ofless than 20 μs after the output voltage reaches the predeterminedthreshold by reducing the period of conduction of the device duringindividual cycles of the radio frequency output signal. Such alterationin the period of conduction is advantageously achieved independently ofany variation in supply voltage to the radio frequency power device. Inpractice, the reduction in output power is brought about using a singlecontrol variable, i.e. the peak output voltage or peak-to-peak outputvoltage, independently of supply voltage and independently of thedelivered output power which varies according to the load impedance andthe supply voltage. Thus, triggering of a power reduction occurs at thesame preset output voltage threshold but at different output power andload impedance values, according to circumstances.

[0009] As an adjunct to direct control of the radio frequency outputstage, the means for causing a reduction in output power may include afurther control line which is coupled to the power supply means, thecontrol circuitry being arranged such that a second power reductionsignal is fed to the power supply means to effect a reduction in theaverage power supply voltage supplied to the output stage. Typically,the rate of reduction of power due to lowering of the power supplyvoltage is comparatively slow, but the combination of two means ofcontrol can produce a larger range of available output power levels.

[0010] In the preferred generator the control circuitry has a firstoutput coupled to a radio frequency power device in the output stage toreduce the radio frequency duty cycle thereof and a second outputcoupled to the power supply to effect a reduction in the average powersupply voltage supplied to the output stage, the said reductionsoccurring in response to the sensing signal reaching a respectivepredetermined threshold value, depending on the mode of treatmentrequired.

[0011] In the case of the power supply being a switched mode powersupply having output smoothing components, the supply circuit may bearranged such that the second power reduction control signal has theeffect of disabling the supply circuit, e.g. by gating the pulsedoutput. Accordingly, a high-speed control response is obtained with thesupply voltage falling relatively slowly after the initial step powerreduction to enable the radio frequency duty cycle of the power deviceto be increased again, thereby allowing further high-speed powerreductions if necessary.

[0012] The technique of directly controlling the radio frequency outputstage can be performed by repeatedly producing, firstly, a rapidreduction in the cycle-by-cycle conduction period of the power devicefrom a peak level to a trough level when the respective output thresholdis reached, followed by, secondly, a progressive increase in theconduction period until the conduction period again reaches its peaklevel, the radio frequency output voltage being monitored during theprogressive increase. This rapid reduction and progressive increasesequence may be repeated until the peak conduction period level can bereached without the output voltage exceeding the respective outputthreshold due to the supply voltage from the switched mode power supplyhaving fallen sufficiently since it was disabled. Re-enabling of thesupply circuit typically occurs after a delay, and conveniently at theend of the first switched mode switching cycle in which the outputvoltage has not reached the threshold for the whole of the switchingcycle. It will be appreciated that, during the blended mode ofoperation, the repeated reduction and restoration of the cycle-by-cycleconduction period of the power device typically occurs many times duringeach period of tissue coagulation or vaporisation. In other words, itoccurs at a much faster rate than the rate of alternation betweenstates.

[0013] The output stage preferably includes an output resonant circuithaving a Q which is sufficiently high to remove switching noise from theswitching device or devices of the stage without unduly slowing theresponse to the output voltage reaching the predetermined threshold.Typically, the Q is sufficient to achieve a crest factor below 1.5, thecrest factor being the ratio of the peak and r.m.s. values of the outputvoltage waveform.

[0014] The generator may have an output impedance in the range of from100 ohms to 250 ohms, and preferably between 130 and 190 ohms. Such agenerator has its radio frequency output stage operable to produce a CW(continuous wave) output, i.e. with a 100% duty cycle or without on/offpulse width modulation at a frequency lower than the r.f. oscillationfrequency. In effect, the output stage may operate as an open loop stagewith a power/load impedance characteristic having a peak (preferably asingle peak) at about 150 to 160 ohms and with the curve decreasingcontinuously with decreasing impedance below the peak and increasingimpedance above the peak.

[0015] Another view of the preferred generator is that of apparatus forsupplying radio frequency power to an electrosurgical instrument foroperation in an electrically conductive fluid medium, the generatorcomprising a radio frequency output stage having a radio frequency powerdevice and at least a pair of electrosurgical output connections for thedelivery of radio frequency power to electrodes, power supply meanscoupled to the output stage, and control circuitry including sensingmeans for deriving a sensing signal representative of the radiofrequency output voltage developed across the output connections, andmeans responsive to the sensing signal for causing a reduction indelivered output power when the sensing signal is indicative of apredetermined output voltage threshold having been reached, wherein thecontrol circuitry is arranged such that the reduction in output power iseffected by reducing the period of conduction of the device duringindividual cycles of radio frequency oscillation, preferablyindependently of the supply voltage to the device.

[0016] The generator has at least a pair of electrosurgical outputconnections for the delivery of radio frequency power to the instrument,means coupled to the output stage for supplying power to the outputstage, and control circuitry including sensing means for deriving asensing signal representative of the radio frequency output voltagedeveloped across the output connections and means responsive to thesensing signal for causing at least a 50% reduction in delivered outputpower when the sensing signal is indicative of a predetermined outputvoltage threshold having been reached, the said reduction being effectedwithin a period of 20 μs or less.

[0017] The invention also includes an electrosurgical system comprisingan electrosurgical generator for generating radio frequency power and anelectrosurgical instrument coupled to the generator, the instrumenthaving an electrode structure for operation immersed in an electricallyconductive liquid, wherein the system has a first mode of operation inwhich tissue is treated by the application of heat in the region of theelectrode structure without forming an electrode-enveloping vapourpocket, and a second mode of operation in which the tissue is locallyvaporised by energy transmitted from the electrode structure via anelectrode-enveloping vapour pocket, said first and second modes beingdefined by different respective electrical control parameters selectedin the generator, and wherein the system has a third, blended mode ofoperation produced by constantly alternating between the first andsecond modes. The electrode structure may include a distal treatmentelectrode and a liquid contact electrode spaced proximally from thedistal electrode, both electrodes being for use surrounded by theconductive liquid and each being connected to a respective one of thepair of output connections the control stage being operable to reducethe reduction time of the power device when the conductive liquid at thedistal electrode is vaporised. The electrosurgical instrument mayprovide an electrode structure having juxtaposed first and secondelectrodes for immersion in the conductive liquid, the first and secondelectrodes respectively forming a tissue treatment electrode at anextreme distal end of the instrument and a return electrode proximallyspaced from the tissue contact electrode.

[0018] The preferred system is operable in a tissue desiccation mode, atissue cutting or vaponsation mode and a blended mode, and comprises agenerator for generating radio frequency power and an electrosurgicalinstrument coupled to the generator, the instrument having an electrodestructure for operation immersed in a conductive liquid, wherein thegenerator includes a mode selection control and has power controlcircuitry for automatically adjusting the radio frequency power suppledto the electrode structure to limit the peak generator output voltage toa first value when the desiccation mode is selected and to at least onesecond value when the cutting or vaporisation mode is selected, thesecond value or values being higher than the first value, and tocontinually alternated first and second values when the blended mode isselected. The first and second values are advantageously in the rangesof from 150V to 200V, and from 250V to 600V respectively, these voltagesbeing peak voltages.

[0019] From a method aspect, the invention provides an electrosurgicalsystem comprising an electrosurgical radio frequency generator coupledto an electrode assembly having a treatment electrode; introducing theelectrode assembly into a selected operation site with the treatmentelectrode contacting the tissue to be treated and with the tissue andthe treatment electrode immersed in a conductive liquid; actuating thegenerator; and controlling the radio frequency power applied to thetreatment electrode by the generator so as constantly to alternatebetween (a) a first treatment state in which the tissue to be treated isheated with the liquid adjacent the electrode maintained substantiallyat its boiling point without creating a vapour layer surrounding theelectrode, and (b) a second treatment state in which said tissue isvaporised via a layer of vapour from the conductive liquid which ismaintained around the electrode without overheating of the electrode,thereby to treat the tissue in a blended mode of operation in which thetissue is vaporised and neighbouring tissue is coagulated. The radiofrequency power supply to the electrode may be automatically adjusted byalternately limiting the output voltage to predetermined first andsecond voltage values, the first voltage value being used fordesiccation and the second voltage value, which is higher than the firstvoltage value, being used for cutting or vaporisation to yield therequired blended effect.

[0020] Alternatively, the output voltage may be modulated to produce apulsed waveform in which bursts of radio frequency power are separatedby periods of zero voltage output. Constant alternation between tissuedesiccation and tissue vaporisation occurs, desiccation being obtainedat the beginning of each burst prior to formation of a vapour pocketaround the treatment electrode, and afterwards due to residual heat inthe treatment electrode. Each time the vapour pocket forms, tissuevaporisation occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention is illustrated by way of example in the drawings inwhich:

[0022]FIG. 1 is a diagram showing an electrosurgical system inaccordance with the invention;

[0023]FIG. 2 is a fragmentary view of a first electrode assembly fortissue desiccation, shown in use and immersed in a conductive liquid;

[0024]FIG. 3 is a load characteristic graph illustrating the variationin load impedance produced by an electrode assembly such as that shownin FIG. 2 when used in a conductive liquid, according to the deliveredoutput power;

[0025]FIG. 4 is a fragmentary view of a second electrode assembly fortissue vaporisation, shown in use immersed in a liquid;

[0026]FIG. 5 is a fragmentary view of a third electrode assembly fortissue cutting, or for combined tissue cutting and desiccation in ablended mode of operation;

[0027]FIG. 6 is a block diagram of a generator in accordance with theinvention;

[0028]FIG. 7 is a block diagram of part of the control circuity of thegenerator of FIG. 6;

[0029]FIG. 8 is a waveform diagram showing a typical RF output voltagevariation pattern obtained with the generator of FIGS. 6 to 8, thevoltage being shown varying with time according to variations in loadimpedance and generator output stage supply voltage;

[0030]FIG. 9 is a circuit diagram of part of the generator of FIGS. 6and 7;

[0031]FIG. 10 is a graph showing the variation of output power producedby the generator as a function of the load impedance presented to it bythe electrode assembly, the output power variation being shown in twooperation modes of the generator; and

[0032]FIG. 11 is a graph showing the variation of output power forgenerator as a function of load impedance after modification of thegenerator characteristics in response to output voltage sensing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0033] Historically, underwater electrosurgery has been the mostdemanding electrosurgical application in terms of instrumentengineering. The reason for this is that the electrosurgical powerrequirement is very high, specifically because it is necessary to createarcs for cutting and tissue disruption in circumstances in which poweris dissipated quickly by the surrounding liquid. Consequently, highcurrents are used to ensure vaporisation of liquid surrounding theelectrode. Power levels up to 300 watts are commonly used.Conventionally, underwater electrosurgery is performed using anon-conductive fluid or irrigant to eliminate electrical conductionlosses. Glycine, which is commonly used, has the disadvantage that inthe course of an operation, veins may become severed and irrigant may beinfused into the circulation. This absorption causes among other thingsa dilution of serum sodium which can lead to a condition known as waterintoxication.

[0034] Accordingly, the applicants propose use of a conductive liquidmedium such as normal saline, electrosurgery being performed with usinga system comprising a generator and an instrurnent, the instrumenthaving a dual-electrode structure with the saline acting as a conductorbetween the tissue being treated and one of the electrodes, hereinaftercalled the “return electrode”. The other electrode is located adjacentto or applied directly to the tissue. This other electrode ishereinafter called the “active electrode”.

[0035] Such a system is shown in FIG. 1. The generator 10 has an outputsocket 10S providing a radio frequency (RF) output for an instrument inthe form of a handpiece 12 via a connection cord 14. Activation of thegenerator may be performed from the handpiece 12 via a controlconnection in cord 14 or by means of a footswitch unit 16, as shown,connected separately to the rear of the generator 10 by a footswitchconnection cord 18. In the illustrated embodiment, footswitch unit 16has two footswitches 16A and 16B for selecting a desiccation mode and avaporisation mode of the generator respectively. The generator frontpanel has push buttons 20 and 22 for respectively setting desiccationand vaporisation power levels, which are indicated in a display 24. Pushbuttons 26 are provided as an alternative means for selection betweendesiccation and vaporisation modes, and as a means of setting a blendedmode.

[0036] Handpiece 12 mounts a detachable electrode assembly 28 having adual electrode structure, as shown in the fragmentary view of FIG. 2.

[0037]FIG. 2 is an enlarged view of the distal end of electrode assembly28. At its extreme distal end the assembly has an active electrode 30which, in this embodiment, is formed as a series of metal filamentsconnected to a central conductor 32. The filaments may be made ofstainless steel. Proximally of the active electrode 30 and spaced fromthe latter by a longitudinally and radially extending insulator 34 is areturn electrode 36. The return electrode 36 is arranged coaxiallyaround the inner conductor 32 as a sleeve 38 which extends as a tubularshaft 40 to the proximal end of the assembly 28 where it is connected inthe handpiece 12 to conductors in the connection cord 14. Similarly, theinner conductor 32 extends to the handpiece and is connected to aconductor in cord 14. The electrode assembly 28 has an insulating sheath42 which covers shaft 40 and terminates proximally of the insulator 34to leave the distal end of shaft 40 exposed as the return electrode 36.

[0038] In operation as a desiccation instrument, the electrode assembly28 is applied as shown in FIG. 2 to the tissue 44 to be treated, theoperation site being immersed in a normal saline (0.9% w/v) solution,here shown as a drop 46 of liquid surrounding the distal end portion ofthe electrode assembly 28. The liquid immerses both the active electrode30 and the return electrode 36.

[0039] Still referring again to FIG. 2, the metallic filaments formingthe active electrode 30 are all electrically connected together and tothe inner conductor 32 of the electrode assembly to form a unitaryactive electrode. Insulator 34 is an insulating sleeve, the distal endportion of which is exposed proximally of the exposed part of the activeelectrode 30. Typically, this sleeve is made of a ceramic material toresist damage from arcing. The return electrode terminates at a pointshort of the end of the insulator 36 so that it is both radially andaxially spaced from the active, or tissue contacts electrode 30. Thesurface area of the return electrode is considerably greater than thatof the active electrode 30. At the distal end of the electrode assembly,the diameter of the return electrode is typically in the region of from1 mm to 3 mm, with the longitudinal extent of the exposed part of thereturn electrode being typically between 1 mm and 5 mm with thelongitudinal spacing from the active electrode being between 1 mm and 5mm.

[0040] In effect, the electrode assembly is bipolar, with only one ofthe electrodes (30) actually extending to the distal end of the unit.This means that the return electrode, in normal circumstances, remainsspaced from the tissue being treated and a current path exists betweenthe two electrodes via the tissue and the conductive liquid which is incontact with the return electrode 36.

[0041] The conductive liquid 46 may be regarded, as far as the deliveryof bipolar electrosurgical energy is concerned, as a low impedanceextension of the tissue. Radio frequency currents produced by thegenerator 10 flow between the active electrode 30 and the returnelectrode 36 via the tissue 44 and the immersing conductive liquid 46.The particular electrode arrangement shown in FIG. 2 is most suitablefor tissue desiccation.

[0042] The axial as well as radial separation between the electrodesavoids the small spacing of the conventional bipolar arrangement inwhich both electrodes are tissue-contacting. As a result, there is lessdanger of unwanted arcing across the insulation surface, which allowscomparatively high power dissipation for desiccation treatment, and, inthe case of tissue cutting or vaporisation, prevents excessive arcingwhich can lead to inter-electrode insulation damage.

[0043] The immersing saline solution may be provided from a conduit (notshown) forming part of the instrument 12. Thus, the invention may takethe form of an electrosurgical system for the treatment of tissueimmersed in a conductive fluid medium, comprising an electrosurgicalinstrument having a handpiece and an instrument shaft, and, on the endof the shaft, an electrode assembly, the assembly comprising a tissuetreatment electrode which is exposed at the extreme distal end of theinstrument, and a return electrode which is electrically insulated fromthe tissue treatment electrode and has a fluid contact surface spacedproximally from the exposed part of the tissue treatment electrode, thesystem further comprising a radio frequency generator coupled to theelectrode assembly of the instrument, a reservoir of electricallyconductive fluid, such as the normal saline solution, and a conduit,typically and integral part of an endoscope, for delivering the liquidfrom the reservoir to the region of the electrode assembly. Pressure fordelivering the liquid may be provided by a pump forming part of theapparatus.

[0044] Since in this embodiment of electrode assembly 28, the activeelectrode 30 is made of stainless steel filaments in the form of abrush, the electrode is flexible, providing a reproducible tissue effectwhich is comparatively independent of the application angle of theelectrode to the tissue surface. The flexibility of the electrode 30also results in a differential contact area of the active electrodedependent on the applied pressure, allowing variations in the breadth ofdesiccation over the surface of the tissue, reducing procedure time.

[0045] Desiccation occurs by virtue of radio frequency currents passingbetween the active electrode 30 and the conductive liquid 46 via theouter layer of the tissue 44 immediately beneath and in an areasurrounding the active electrode 30. The output impedance of thegenerator is set at a level commensurate with the load impedance of theelectrode assembly when used as shown in FIG. 2 with both electrodes incontact with the conductive liquid 46. In order to sustain this matchedstate for tissue desiccation, the output power of the generator isautomatically controlled in a manner which will be described below sothat vapour bubbles of significant size are substantially prevented fromappearing at the active electrode 30, thereby avoiding a consequentincrease in load impedance. In this way, the active electrode can becontinually wetted by the conductive liquid so that, whilst the tissuewater is removed by thermal desiccation, the impedance reaches an upperlimit corresponding to the point at which the conductive liquid startsto boil. As a result, the system is able to deliver high power levelsfor desiccation without unwanted conductive liquid vaporisation leadingto unwanted tissue effects.

[0046] The electrical behaviour of the electrode assembly when theelectrodes 30 and 36 are immersed in the conductive liquid 46 is nowconsidered with reference to the graph of FIG. 3.

[0047] When power is first applied, there is presented to the generatoran initial load impedance r which is governed by the geometry of theelectrode and the electrical conductivity of the conductive liquid. Thevalue of r changes when the active electrode touches the tissue. Thehigher the value of r, the greater is the propensity of the conductiveliquid to vaporise. As power is dissipated in the tissue and theconductive liquid, the conductive liquid increaes in temperature. In thecase of normal saline, the temperature coefficient of conductivity ispositive and the corresponding impedance coefficient is thereforenegative so that the impedance initially falls. Thus, the curve in FIG.3 indicates a fall in load impedance as the delivered power isincreased, the impedance falling through point A to a minimum at pointB, at which point saline in immediate contact with the electrode reachesboiling point. Small vapour bubbles now form on the surface of theactive electrode and the impedance starts to rise as shown by the curverising from point B to point C. Thus, once the boiling point has beenreached, the arrangement displays a dominant positive power coefficientof impedance so that small increases in power now bring about largeincreases in impedance.

[0048] As the vapour bubbles form, there is an increase in the powerdensity at the remaining active electrode to saline interface (theexposed area of the active electrode not covered by vapour bubbles)which further stresses the interface, producing more vapour bubbles andthus even higher power density. This is a runaway condition, with anequilibrium point only occurring once the electrode is completelyenveloped in vapour. The only means of preventing the runaway conditionis to limit applied voltage, thereby preventing power dissipation intohigher impedance loads. Thus, for a given set of variables, there is apower threshold corresponding to point C at which this new equilibriumis reached.

[0049] In the light of the foregoing, it will be appreciated that theregion between points B and C in FIG. 3 represents the upper limit ofdesiccation power which can be achieved. The transition from point “C”in the vaporise equilibrium state will follow the power impedance curvefor the RF stage of the generator (shown as a dotted line in FIG. 3).

[0050] Upon formation of an electrode-enveloping vapour pocket, theimpedance elevates to about 1 kΩ, as shown by point D in FIG. 3, theactual impedance value depending on a number of system variables. Thevapour is then sustained by discharges across the pocket between theactive electrode and the vapour/saline interface.

[0051] This state of affairs is illustrated by the diagram of FIG. 4which shows an alternative electrode assembly 28A having a hemisphericalor ball electrode 30A in place of the brush electrode 30 of theembodiment of FIG. 2. As before, the return electrode 36A is proximallyspaced from the active electrode 30A by an intervening insulator 34A.The ball electrode is preferred for tissue vaporisation.

[0052] Once in the vaporisation equilibrium state, the vapour pocket,shown by the reference 50 in FIG. 4, is sustained by discharges 52across the vapour pocket between the active electrode 30A and the vapourto saline interface. The majority of power dissipation occurs withinthis pocket with consequent heating of the active electrode. The amountof energy dissipation in this conduction is a function of the deliveredpower. It will be noted from FIG. 3 that the vaporisation mode,indicated by the dotted boundary lines, can be sustained at much lowerpower levels than are required to bring about formation of the vapourpocket. The impedance/power characteristic consequently displayshysteresis. Once the vaporisation mode has been established, it can bemaintained over a comparatively wide range of power levels, as shown bythe inclined part of the characteristic extending on both sides of pointD. However, increasing the delivered output power beyond thatrepresented by point D causes a rapid rise in electrode temperature,potentially damaging the electrode. It should be noted that, if powerwere delivered at the same level as point “C”, the resulting voltageswould cause electrode destruction. The normal operating point for anelectrode used for vaporisation is illustrated by the point “D”. Thispoint is defined uniquely by combination of the impedance powercharacteristic for the electrode in conjunction with the vaporisevoltage limit. To collapse the vapour pocket and to return todesiccation mode requires a significant power reduction back to point A,direct contact between the active electrode and the saline beingreestablished and the impedance falling dramatically. The power densityat the active electrode also falls so that the temperature of the salinenow falls below boiling point and the electrode is then once again in astable desiccation equilibrium.

[0053] In a third, blended mode of operation, the vapour pocketcontinually and successively forms, collapses and reforms as thegenerator is controlled so as to alternate between the electricalconditions required for coagulation or desiccation and those requiredfor tissue cutting or vaporisation. The blended mode is particularlyuseful for procedures involving vascular tissue removal, the combinedtissue cutting and desiccation effect largely preventing blood loss.

[0054] Referring to FIG. 5, an electrode assembly which may typically beused in the blended mode has an exposed treatment portion 30B in theform of a rigid needle, bent to form a hook. As in the other electrodeassemblies described above, the fluid contact portion of the returnelectrode 36B is set back from the active electrode in the direction ofa treatment axis 52. One dimensional characteristic of this electrode isthat the ratio of (i) the length of the shortest conductive path betweenthe return electrode 36B and the furthermost point of the activeelectrode treatment portion 30B (shown as “b” in FIG. 5) and (ii) thelength of the shortest conductive path between the return electrode 36Band the treatment portion 30B (shown as “a” plus “c” in FIG. 5), is inthe range of from 1.25:1 and 2:1. The applicants have found that optimumvalues for this ratio tend to be 1.5:1 or 1.6:1 upwards.

[0055] In use, the hooked end of the treatment portion 30B is applied tothe surface of tissue to be severed and dragged along to produce a cutline. As this movement is executed, tissue is vaporised during theperiod when a vapour pocket is present over the active electrode, andsurrounding tissue is then desiccated when the pocket collapses, therebyreducing bleeding.

[0056] The generator to be described hereinafter has the ability tosustain both the desiccation mode and the vaporisation mode, and tooscillate between the two in the blended mode. Whilst in general theelectrode assemblies illustrated in FIGS. 2 and 4 can be used in eithermode, the brush electrode of FIG. 2 is preferred for desiccation due toits wide potential area of coverage, and the ball electrode of FIG. 4 ispreferred for vaporisation due to its small active electrode/returnelectrode surface area ratio. Although the blended mode can be used witheither of these electrode assemblies, that of FIG. 5 has particularadvantages as will be clear from the preceding description. As can beseen from FIG. 4, during the cut or vaporisation mode, tissuevaporisation occurs when the vapour pocket 50 intersects the tissuesurface, with the electrode assembly preferably being held spaced abovethe tissue surface by a small distance (typically 1 mm to 5 mm).

[0057] The runaway condition which occurs when the delivered powerreaches the level shown by point C in FIG. 3 is exacerbated if thegenerator has a significant output impedance, because the output voltagecan then suddenly rise. With increased power dissipation and without thepresence of the cooling liquid around the active electrode 30, theelectrode temperature rises rapidly with consequent damage to theelectrode. This also produces uncontrollable tissue disruption in placeof the required desiccation. For this reason, the preferred generatorhas an output source impedance which, approximately at least matches theload impedance of the electrode structure when wetted.

[0058] The preferred generator now to be described allows bothdesiccation electrosurgery substantially without unwanted celldisruption, and electrosurgical cutting or vaporisation substantiallywithout electrode burning. Although intended primarily for operation ina conductive liquid distension medium, it has application in otherelectrosurgical procedures, e.g. in the presence of a gaseous distensionmedium, or wherever rapid load impedance changes can occur.

[0059] Referring to FIG. 6, the generator comprises a radio frequency(RF) power oscillator 60 having a pair of output connections 60C forcoupling via output terminals 62 to the load impedance 64 represented bythe electrode assembly when in use. Power is supplied to the oscillator60 by a switched mode power supply 66.

[0060] In the preferred embodiment, the RF oscillator 60 operates atabout 400 kHz, with any frequency from 300 kHz upwards into the HF rangebeing feasible. The switched mode power supply typically operates at afrequency in the range of from 25 to 50 kHz. Coupled across the outputconnections 60C is a voltage threshold detector 68 having a first output68A coupled to the switched mode power supply 16 and a second output 68Bcoupled to an “on” time control circuit 70. A microprocessor controller72 coupled to the operator controls and display (shown in FIG. 1), isconnected to a control input 66A of the power supply 66 for adjustingthe generator output power by supply voltage variation and to athreshold-set input 68C of the voltage threshold detector 68 for settingpeak RF output voltage limits.

[0061] In operation, the microprocessor controller 72 causes power to beapplied to the switched mode power supply 66 when electrosurgical poweris demanded by the surgeon operating an activation switch arrangementwhich may be provided on a handpiece or footswitch (see FIG. 1). Aconstant or alternating output voltage threshold is set via input 68Caccording to control settings on the front panel of the generator (seeFIG. 1). Typically, for desiccation or coagulation the threshold is setat a desiccation threshold value between 150 volts and 200 volts. When acutting or vaporisation output is required, the threshold is set to avalue in the range of from 250 or 300 volts to 600 volts. These voltagevalues are peak values. Their being peak values means that fordesiccation at least it is preferable to have an output RF waveform oflow crest factor to give maximum power before the voltage is clamped atthe values given. Typically a crest factor of 1.5 or less is achieved.

[0062] When a blended output is required, the voltage threshold set viainput 68C is constantly alternated between the value for desiccation orcoagulation and the value for cutting or vaporisation.

[0063] When the generator is first activated, the status of the controlinput 601 of the RF oscillator 60 (which is connected to the “on” timecontrol circuit 70) is “on”, such that the power switching device whichforms the oscillating element of the oscillator 60 is switched on for amaximum conduction period during each oscillation cycle. The powerdelivered to the load 64 depends partly on the supply voltage applied tothe RF oscillator 60 from the switched mode power supply 66 and partlyon the load impedance 64. If the supply voltage is sufficiently high,the temperature of the liquid medium surrounding the electrodes of theelectrosurgical instrument (or within a gaseous medium, the temperatureof liquids contained within the tissue) may rise to such an extent thatthe liquid medium vaporises, leading to a rapid increase in loadimpedance and a consequent rapid increase in the applied output voltageacross terminals 12. This is an undesirable state of affairs if adesiccation output is required. For this reason, the voltage thresholdfor a desiccation output is set to cause trigger signals to be sent tothe “on” time control circuit 70 and to the switched mode power supply66 when the threshold is reached. The “on” time control circuit 70 hasthe effect of virtually instantaneously reducing the “on” time of the RFoscillator switching device. Simultaneously, the switched mode powersupply is disabled so that the voltage supplied to oscillator 60 beginsto fall.

[0064] Subsequent control of the “on” time of individual cycles of theoscillator 60 will be understood by considering the internalconfiguration of the “on” time control circuit 20 which is shown in FIG.7. The circuit comprises an RF sawtooth generator 74 (synchronised atthe RF oscillation frequency by a synchronisation signal derived fromthe oscillator and applied to a synchronisation input 74I), and a rampgenerator 76 which is reset by a reset pulse from the output 68B of thevoltage threshold detector 68 (see FIG. 6) produced when the setthreshold voltage is reached. This reset pulse is the trigger signalreferred to above. The “on” time control circuit 70 further comprises acomparator 78 for comparing the sawtooth and ramp voltages produced bythe sawtooth and ramp generators 74 and 76 to yield a square wavecontrol signal for application to the input 601 of the RF oscillator 60.As shown by the waveform diagrams in FIG. 7, the nature of the sawtoothand ramp waveforms is such that the mark-to-space ratio of the squarewave signal applied to the oscillator 60 progressively increases aftereach reset pulse. As a result, after a virtually instantaneous reductionin “on” time on detection of the output voltage reaching the set voltagethreshold, the “on” time of the RF oscillator is progressively increasedback to the original maximum value. This cycle is repeated until thesupply voltage for the oscillator from power supply 66 (FIG. 6) hasreduced to a level at which the oscillator can operate with the maximumconduction period without the output voltage breaching the set voltagethreshold as sensed by the detector 68.

[0065] The output voltage of the generator is important to the mode ofoperation. In fact, the output modes are defined purely by outputvoltage, specifically the peak output voltage. The absolute measure ofoutput voltage is only necessary for multiple term control. However, asimple term control (i.e. using one control variable) can be used inthis generator in order to confine the output voltage to predeterminedlimit voltages. Thus, the voltage threshold detector 68 shown in FIG. 6compares the RF peak output voltage with a preset DC threshold level,and has a sufficiently fast response time to produce a reset pulse forthe “on” time control circuit 70 within one RF half cycle.

[0066] Before considering the operation of the generator further, it isappropriate to refer back to the impedance/power characteristic of FIG.3. It will be appreciated that the most critical control threshold isthat applicable during desiccation. Since vapour bubbles forming at theactive electrode are non-conducting, the saline remaining in contactwith the electrode has a higher power density and consequently an evengreater propensity to form vapour. This degree of instability bringsabout a transition to a vaporisation mode with the same power level dueto the runaway increase in power density at the active electrode. As aresult, the impedance local to the active electrode rises. Maximumabsorbed power coincides with the electrode condition existingimmediately before formation of vapour bubbles, since this coincideswith maximum power distribution and the greatest wetted electrode area.It is therefore desirable that the electrode remains in its wetted statefor the maximum desiccation power. Use of voltage limit detection bringsabout a power reduction which allows the vapour bubbles to collapsewhich in turn increases the ability of the active electrode to absorbpower. For this reason, the generator described in this specificationincludes a control loop having a large overshoot, in that the feedbackstimulus of the peak voltage reaching the predefined threshold causes alarge instantaneous reduction in power. This control overshoot ensures areturn to the required wetted state.

[0067] In the generator described above with reference to FIGS. 6 and 7,power reduction in response to voltage threshold detection takes placein two ways:

[0068] (a) an instantaneous reduction in RF energy supplied to theresonant output circuit of the oscillator, and

[0069] (b) a shut down of DC power to the oscillator for one or morecomplete cycles of the switched mode power supply (i.e. typically for aminimum period of 20 to 40 μs).

[0070] In the preferred embodiment, the instantaneous power reduction isby at least three quarters of available power (or at least half voltage)from the DC power supply, but continuous voltage threshold feedbackcontinually causes a reduction in delivered power from the DC powersupply. Thus, a high speed response is obtained in the RF stage itself,with the DC supply voltage tracking the reduction to enable the RF stageto return to a full duty cycle or mark-to-space ratio, thereby enablingfurther rapid power reductions when the voltage threshold is againbreached.

[0071] The effect of this process on the RF output voltage is shown inthe waveform diagram of FIG. 8, containing traces representative of theoutput voltage, the oscillator supply voltage, and the load impedanceduring a typical desiccation episode over a Ims period.

[0072] Starting on the lefthand side of the diagram with the supplyvoltage approximately constant, the output voltage increases withincreasing load impedance to a point at which the output voltagethreshold is reached, whereupon the above-described instantaneousreduction in oscillator “on” time occurs. This produces a rapid decreasein the RF output voltage, as shown, followed by a progressive increase,again as described above. When the output voltage reaches the thresholdvoltage, the voltage threshold detector 68 (shown in FIG. 6) alsodisables the power supply, leading to a gradual decrease in the supplyvoltage. As a result. when the “on” time of the oscillator device hasonce again reached its maximum value, illustrated by point a in FIG. 8,the threshold voltage has not been reached. However, the load impedancebegins rising again, causing a further, albeit slower, increase in theoutput voltage until, once again, the threshold voltage is reached(point b). Once more, the “on” time of the oscillator is instantlyreduced and then progressively increased, so that the output voltagewaveform repeats its previous pattern. Yet again, the threshold voltageis reached, again the output voltage is instantly reduced (at point c),and again the “on” time is allowed to increase. On this occasion,however, due to the supply voltage having further reduced (the powersupply still being disabled), the output voltage does not reach thethreshold level (at point d) until a considerably longer time period haselapsed. Indeed, the length of the period is such that the outputvoltage has failed to reach the threshold voltage over a completeswitching cycle of the power supply, so that it has in the meantime beenenabled (at point e).

[0073] During this period the power supplied to the electrode has beensufficient to further increase the load impedance. The erratic impedancebehaviour is typical of the commencement of vapour formation.Consequently, when the threshold voltage is next reached (at point e),several successive cycles of “on” time reduction and increase occurringone after the other are required (see f) combined with a furtherdisabling (see g) of the power supply in order to maintain the voltagebelow the threshold.

[0074] It will be seen, then, that the control circuitry 70, 72 (FIG. 6)operates dynamically to control the output voltage both sufficientlyrapidly and to a sufficient degree to maintain the voltage at a levelconsistent with, in this case, the level required for desiccationwithout tissue disruption due to arcing. The same technique can be usedwith a different threshold voltage to limit the output voltage toprevent electrode burning and/or excessive tissue vaporisation. In thelatter case, the voltage limit may be set to a level between 250 volts(preferably 300 volts) and 600 volts.

[0075] Due to the high power density at the active electrode during thevaporisation mode, the great majority of delivered power is dissipatedin the proximity of the electrode. In the vaporisation mode, it isdesirable that a minimum of saline heating occurs, but that any tissuewhich encroaches the vapour boundary of the active electrode isvaporised. In the vaporisation mode, the vapour is sustained by arcswithin the vapour pocket as described above with reference to FIG. 4.Increasing the output voltage during vaporisation results in increasedvolume of tissue removal due to the increased size of the vapour pocket.Collapse of the vapour pocket during tissue vaporisation has greaterconsequence, due to the increased necrosis as a result of the greaterpower dissipation in the surrounding saline. Vapour pocket collapse canbe prevented by, firstly, arranging for the electrode impedance invaporisation mode to be such that the instrument is in an unmatchedcondition as regards impedance, with result that the resonant outputcircuit Q is high and the output voltage does not change so rapidly aswith lower load impedances and, secondly, the active electrode has asignificant heat capacity that sustains the vapour pocket for asignificant period.

[0076] An unwanted increased in the size of the vapour pocket can beprevented by limiting the peak output voltage during the vaporisationmode, which may be conveniently carried out by substituting a differentthreshold value for the voltage threshold detector 68 (see FIG. 6) whenin the vaporisation mode.

[0077] The circuitry of the RF oscillator 60, voltage threshold detector68. and “on” time control circuit 70 (shown in FIG. 6) in the preferredgenerator in accordance with the invention is shown in FIG. 8.

[0078] Referring now to FIG. 9, the RF oscillator comprises a IGBT(insulated gate bipolar transistor) 80 acting as an RF switching devicewhich pumps energy into a parallel resonant circuit comprising theprimary winding 82P of transformer 82 and a parallel-connectedresonating capacitor 84. RF power is supplied from the transformersecondary winding 82S via isolating capacitors 86, 88 to RF outputterminals 62. Power for the oscillator transistor 80 is supplied on ahigh voltage supply line 90 which is connected to the output of theswitched mode power supply 66 (shown in FIG. 6). Supply line 90 isdecoupled by capacitor 92.

[0079] The oscillator feedback loop runs from the resonant primarywinding 82P (on the opposite side of the winding from the supply line90) via a phase shift network comprising capacitor 94, resistor 96, andclamping diodes 98, 100, and via a field effect transistor (FET) 104,the voltage controlled monostable represented by comparator 78 andassociated components, and the driver 108, which is connected to thegate of transistor 80.

[0080] The voltage on that side of the primary winding 82P which iscoupled to transistor 80 is substantially sinusoidal and alternates at afrequency defined by the parallel resonant combination of the windinginductance and capacitor 84. Typically the voltage swing is greater thantwice the supply voltage on supply line 90, falling below ground voltagein negative half-cycles.

[0081] The phase-shift network 94, 96, 98, 100 provides a positive-goingsquare wave which is 90° phase-advanced with respect to the primaryvoltage. Thus, FET 104 is turned on approximately when the voltage onprimary winding 82P has just reached its minimum value, and off when ithas just reached its maximum value. When FET 104 is turned on a timingcapacitor is rapidly discharged and the output of comparator 78 isturned off. The driver 108 is non-inverting and consequently transistor80 is also turned off at this point. It follows that the transistor“off” point is repeatable and has a constant phase relationship withrespect to the primary voltage by virtue of the feedback path describedabove. The logic of the feedback path is also such that the feedbacksignal fed to the gate connection of transistor 80 has a logic level of“1” when the primary voltage is decreasing (and the potential differenceacross the primary winding 82P is increasing). The “off” point occurssubstantially at a primary voltage peak, i.e. when the primary voltageis at its minimum value in the present case.

[0082] Unlike the “off” point, the “on” point of transistor 80 isvariable as will now be described. The instant at which the logic levelat the output of comparator 78 and on the base of device 80 changes to“1” depends on the reference voltage applied to the inverting input 78Iof comparator 78. As a result, the delay between device 80 switching offand switching on is determined by this comparison of voltage applied toinput 78I of comparator 78. In other words, an “on” signal to device 80is delayed with respect to switching off by a period which is inaccordance with the reference voltage on the inverting input. Thisreference voltage is dependent on the voltage appearing across resistor112 which is part of a potential divider consisting also of resistor 114and potentiometer 116. Potentiometer 116 sets the minimum switching ondelay, corresponding to the maximum duty cycle of transistor 80. Thevoltage appearing across resistor 112 is variable and represents thecontrol range of “on” time adjustment between 25% of the maximum dutycycle and 100%. Timing capacitor 110 is charged by variable resistor 118(preset for an appropriate time constant) from a low voltage supply line120.

[0083] Comparing FIG. 9 with FIG. 7, it will be appreciated that thevoltage on the non-inverting input 78N of comparator 78 has a sawtoothwaveform as shown in FIG. 7, the waveform being produced by the repeatedtriggering of FET 104 and discharging of capacitor 110, each dischargingbeing followed by charging of a capacitor through resistor 118.

[0084] The voltage across resistor 112 is normally at a minimum value,and is increased when the RF output voltage from the generator reaches apredetermined peak threshold value. The circuitry which brings aboutthis effect will now be described.

[0085] Output voltage detection is provided by the capacitive dividerchain 122, 124 connected across the RF output, the tap between thecapacitors feeding the primary winding of an isolating transformer 126.Resistors 128 and 130 connected across the primary and secondarywindings of transformer 126 respectively provide damping to avoidunwanted resonances and to filter high frequency components which mayoccur during arcing at the active electrode. The resulting sensingvoltage appearing at the secondary winding of transformer 126 is thenfed to two comparators 132 and 134. At this point, it should beappreciated that only the positive-going half cycles of the sensevoltage are used for peak output voltage threshold detection.

[0086] Each comparator 132, 134 has two inputs, one connected to thetransformer 126 to receive the sense voltage, and one connected to arespective reference voltage input 136, 138 (labelled CLAMP and BOOST inFIG. 9). Reference voltages applied to these inputs 136, 138 arecomputer generated set voltage thresholds for the desiccate andvaporisation modes respectively. Selection of the operating mode isbrought about by a control signal (MODE) applied to control inputs 140,and the logic chain comprising decoder 141, and gates 142, 144, 146, and148. Mode selection signals applied at inputs 140 cause operation of thedecoder 141 such that in the desiccation mode a logic level “1” is seton the input to gate 142. In vaporisation mode, logic level “0” is seton this input, effectively disabling the output of comparator 132 viaNOR gate 144, the output threshold detection then being fed through NORgate 146. It will therefore be appreciated that the CLAMP voltageapplied to input 136 is the reference voltage setting the thresholdvalue for the peak output voltage during desiccation, while the BOOSTvoltage applied to input 138 sets the threshold value of the peak outputvoltage in the vaporisation mode. When a blended output is signalled,the decoder 141 produces a square wave output, causing togging betweenlogic levels “1” and “0” at a rate which is appropriate for theconnected electrode assembly, which may be between 5 Hz and 2 kHz, andmore typically between 25 Hz and 1 kHz. The actual rate of alternationis determined by the rates at which the vapour pocket forms andcollapses for any given electrode. A larger electrode assembly tends torequire an alternation rate towards the lower end of the 25 Hz to 1 kHzrange, while a small electrode assembly may be able to operateeffectively in a blended mode at 1 kHz. When the electrode assembly isoperating properly in blended mode the repeated formation and collapseof the vapour pocket is audible as a buzz or whine, depending on thefrequency of alternation.

[0087] When the output voltage reaches the set threshold value (i.e. a“limit” voltage), transistor 150 is switched on. This transistor iscapable of charging capacitor 152 from 1.5V to 4V in a period of 50 ns.The base charge of transistor 150 is sufficient to enlarge very narrowpulses from the voltage detection circuitry and therefore ensures thatcapacitor 152 attains maximum voltage for only marginly detected limitvoltages at the RF output. The function of capacitor 152 is to provideprogressively lower reference voltages for comparator 78 after a limitvoltage detection. Thus, the voltage on the emitter of transistor 150has a waveform as shown at the output of the ramp generator 76 in FIG.7. In this way, the turn-on instant of device 80 is instantly retardedwhen the RF output voltage reaches the preset threshold value, and issubsequently progressively advanced as the voltage across resistor 112slowly decreases. The discharge rate of capacitor 152 is determined bythe parallel combination of resistor 112 in parallel with resistor 114plus resistor 116.

[0088] Switching energy provided by transistor 80 is converted by aseries inductor 154P into a current drive into the resonant primarywinding 82P. The action of series inductor 154P smoothes energyinjection into the resonant output circuit represented by winding 82Pand capacitor 84 and prevents excessive initial current throughtransistor 80, and excessive swinging of the voltage input to winding82P above the voltage on supply line 90.

[0089] Under full power conditions, the initial switch-on of transistor80 occurs at an initial resonant voltage maximum across the resonantcircuit. This creates a switch-on current zero as the inductor 154P iscompletely depleted of energy after each cycle. Current in this inductorrapidly builds up until a point is reached at which the voltage onwinding 82P becomes negative. The inductor 154P then releases its energyinto this reverse bias. The current zero at switch-off is thenguaranteed by a blocking diode 156 which prevents the return of energyfrom the resonant circuit to the inductor 154P.

[0090] When the switch-on time of transistor 80 is reduced due to theoutput voltage reaching the predetermined set threshold, the primaryvoltage amplitude across winding 82P decreases to the extent that theprimary peak amplitude is less than the supply voltage. In particular,the voltage minimum at the end of primary winding 82P coupled totransistor 80 no longer swings beyond the ground voltage. Energy can nowno longer be released from inductor 154P back into the resonant circuit.A secondary path for stored energy in inductor 154P is provided by thefact that this inductor is the primary winding of a transformer 154which has a second winding 154S coupled via a diode 158 to the supplyline 90. Residual energy stored in inductor 154P at switch-off causesforward biasing of diode 158 through which the energy is recovered backinto the supply. This recovery mechanism permits partial resonantprimary amplitude levels without damaging switching transistor 80 byuncoupled energy creating excessive voltage.

[0091] The relationship between “on” time of transistor 80 and switchingenergy depends on a number of variables such as the initial energystorage of the resonant circuit 82P, 84, the loading on the outputterminals 62 (which affects the Q of the resonant circuit), and theloading as it affects oscillation frequency, which all affect thenon-linear energy storing rate of inductor 154P.

[0092] As has been described above, detection of the output voltagereaching a predetermined threshold value not only causes the duty cycleof the switching transistor 80 to be instantly reduced, but it alsodisables the switched mode power supply 66 (shown in FIG. 6). Thisdisabling effect is produced by feeding a signal from the output of thelogic chain 142 to 148 via a filter 160 to remove RF transients to aDISABLE output 68A, which is connected to the switched mode power supply66.

[0093] The generator output impedance is set to about 160 ohms. Theeffect of this choice will be evident from the following descriptionwith reference to FIGS. 10 and 11 which are graphs showing the variationof output power which can be produced by the generator into differentload impedances.

[0094] Referring to FIG. 10, the power delivered to the load is hereshown as a function of load impedance for two different oscillatorsupply voltage settings. In both cases, it will be seen that, to theleft of the power/impedance peak, an increase in load impedance leads toan increase in output power and, hence, an increase in output voltage.At higher impedances, to the right of the peaks, the voltage continuesto increase, albeit less aggressively, as impedance increases.

[0095] One of the features of the preferred generator in accordance withthe invention is that the output stage operates as an open looposcillator with an output impedance (corresponding to the peaks in FIG.10) of about 160 ohms. This is considerably lower than the outputimpedance of conventional generators used for underwater electrosurgery,and contributes to the ability to prevent runaway arcing behaviour andconsequent excessive tissue damage and electrode burn-out.

[0096] It should be understood that for desiccation, steam envelopegeneration at the electrode and arcing should be prevented. Conversely,for cutting or vaporisation, steam envelope generation and arcing arerequired, but to a level consistent with achieving the required tissueeffect and the avoidance of electrode burn-out. Operating points for lowand high power desiccation and cutting or vaporisation are shown in FIG.10.

[0097] A feature of the combination of the generator in accordance withthe invention and an electrode assembly having two adjacent electrodesis that the output is virtually bistable. When operating in desiccationmode, the entire electrode surface is in contact with an electricallyconductive medium and therefore the load impedance is comparatively low,consequently inhibiting the rise in output voltage to a level sufficientfor arcing. Conversely, when in cutting or tissue vaporisation mode, theentire active electrode surface is covered with a layer of vapour whichis of much higher impedance, and the vapour pocket is sustained byarcing within it so that nearly all of the power dissipation occurswithin the vapour envelope. In order to traverse from a desiccation modeto the cutting mode, a high power burst is required, hence thepositioning of the power/load curve peak between the desiccation andcutting operation points on the curve. By allowing the output power toincrease with impedance in this way, a high power burst of sufficientenergy to create arcing is achieved despite the low impedance presentedby the electrodes. As the supply voltage to the oscillator is increased,it has a greater propensity to flip into the cut mode, whilst at lowersupply voltage levels, the bistable nature of the output, although morepronounced, tends towards the desiccation state.

[0098] The bistable properties arise not only from the electrodeimpedance behaviour, but also from the shape of the power/load impedancecurve. The flatter the load curve, the more constant the output poweracross a band of impedances and the less pronounced the effect.

[0099] Referring to FIG. 10, it will be appreciated that in the cut ortissue vaporisation mode, a power equilibrium point is achieved byvirtue of the decreasing output power as impedance increases. In thedesiccation mode, the equilibrium is less straightforward, because thereare two impedance change mechanisms. The first mechanism is the heatingof the conductive medium and/or tissue which, due its positivecoefficient of conductivity, results in a falling impedance initially,so that when power is first applied, the operating point moves towardsthe lefthand side of the diagram in FIG. 10. Consequently, there is awell-defined equilibrium point defined by the reduction in impedancewith increasing power supply voltage, and the consequent reduction indelivered output power. However, when the saline medium or tissue fluidsin contact with the active electrode start to boil, small water vapourbubbles begin to form which increase the impedance. When the generatoris about to flip into the cutting mode, impedance rise due to steamformation is dominant. The impedance change therefore becomes positivewith increasing supply voltage, and the operating point moves towardsthe righthand side of the diagram, which allows greater input power as aresult of the shape of the load curve, causing a rapid change to cuttingor vaporisation mode. As steam formation continues to increase, theincreasing impedance causes a fall-off in delivered output power.

[0100] The applicants have found that the inherent equilibria describedabove may be insufficient to maintain a stable coagulation state or astable cutting state. It is for this reason, that the RF output voltagefrom the RF oscillator 60 (FIG. 6) is limited, the limiting occurringextremely rapidly, typically with a response time of 20 μs or less.Excessive radio frequency interference is avoided by linear variation ofthe oscillator switching device “on” time in response to a feedbacksignal from the voltage threshold detector. This technique is used inconjunction with the RF oscillator having a comparatively low output Qwhen matched to the load, this Q being sufficient to suppress switchingnoise without inordinately damping the response to output voltagethreshold detection.

[0101] By way of example, the effect of voltage threshold control for aparticular electrode configuration is shown in FIG. 11. The heavy lines200, 202 indicate the modified power/load impedance characteristics. Fordesiccation, shown by line 200, the switched mode power supply is set toproduce a peak (matched) open loop output power of between 75 watts and110 watts, with the actual peak power in this case being about 90 watts.For cutting and vaporisation (shown by line 202), the peak power can bebetween 120 watts and 175 watts. In this case it is 150 watts. Asexamples, the voltage thresholds are set at 180 volts peak fordesiccation and 300 volts peak for cutting, as illustrated by thehyperbolic constant voltage lines 204 and 206 respectively. Thepower/impedance curves follow the respective constant voltage thresholdlines to the right of their intersection with the unmodified open loopcurves 208 and 210. Thus, it will be understood that the desiccationthreshold line represents the maximum voltage that can be achieved inthe desiccation mode before arcing is produced, whilst the cut thresholdline limits the cutting or tissue vaporisation performance to achievethe desired tissue effect and. in the extreme, to avoid electrodeburn-out. The desiccation threshold line also represents a voltageinsufficient to achieve arcing for cutting or vaporising tissue.

[0102] A significant feature of the generator characteristic forelectrosurgical cutting or tissue vaporisation is that at peak power(matched impedance) the load impedance lies between the impedancescorresponding to the threshold voltages at that power level. Incontrast, in the desiccation mode, the power/load impedancecharacteristic has a power peak at an impedance lying below thedesiccation threshold line at that power level.

[0103] In practice, the output power in the desiccation mode will behigher than in the cutting or tissue vaporisation mode. The reason forthis statement (despite the apparent contradiction with the load curvesin FIG. 11) is that the equilibrium points described above lie atdifferent points on the respective curves. To ensure cutting, the highpeak power of the higher curve is required to reach the cut thresholdline (corresponding to 300 volts peak). The cutting mode then followsthe cutting or vaporisation threshold line. The cutting operating pointis defined by the load impedance created when a suitable level of arcingis occurring. Typically, the load impedance in these circumstances isgreater than 1000 ohms. Thus, although a full 150 watt peak power isavailable to ensure that vapour pockets are formed to promote arcing forcutting, the actual power drawn during cutting or tissue vaporisationfor this particular electrode example may be between 30 watts and 40watts. This situation is more easily understood if reference is alsomade to FIG. 3.

[0104] In the desiccation mode, the operating point is determined by thepositive power coefficient of impedance arising from steam generation.Consequently, the equilibrium naturally occurs in the region of the peakof the desiccation mode power/load impedance curve.

[0105] The invention is useful for dissection, resection, vaporisation,desiccation and coagulation of tissue and such combinations of thesefunctions with particular application in hysteroscopic, laparoscopic,colposcopic (including vaginal speculum) and open surgical procedures onthe female genital tract and adnexal related diseases. Hysteroscopicoperative procedures may include: removal of submucosal fibroids, polypsand malignant neoplasms; resection of congenital uterine anomalies suchas a septum or subseptum; division of synechiae (adhesiolysis); ablationof diseased or hypertrophic endometrial tissue; and haemostasis.Laparoscopic operative procedures may include: removal of subserosal andpedunculated fibroids, ablation of ectopic endometrium, ovariancystectomy and ovarian drilling procedures; oophorectomy,salpingo-oophorectomy, subtotal hysterectomy and laparoscopicallyassisted vaginal hysterectomy (LAVH) as may be performed for benign ormalignant diseases laparoscopic uterosacral nerve ablation (LUNA);fallopian tube surgery as correction of ectopic pregnancy orcomplications arising from acquired obstructions; division of abdominaladhesions; and haemostasis.

[0106] The invention is also useful in the lower female genital tract,including treatment of the cervix, vagina and external genitalia whetheraccessed directly or using instrumentation comprising generally speculaeand colposcopes. Such applications include: vaginal hysterectomy andother pelvic procedures utilising vaginal access; LLETZ/LEEP procedure(large loop excision of the transformation zone) or excision of thetransformation zone of the endocervix; removal of cystic or septiclesions; ablation of genital or venereal warts; excision of benign andmalignant lesions; cosmetic and surgical repairs including vaginalprolapse; excision of diseased tissue; and haemostasis.

[0107] The invention is also useful for dissection, resection,vaporisation, desiccation and coagulation of tissue and suchcombinations of these functions with particular application inarthroscopic surgery as it pertains to endoscopic and percutaneousprocedures performed on joints of the body including but not limited tosuch techniques as they apply to the spine and other non-synovialjoints. Arthroscopic operative procedures may include: partial orcomplete meniscectomy of the knee joint including meniscal cystectomy;lateral retinacular release of the knee joint; removal of anterior andposterior cruciate ligaments or remnants thereof; labral tear resection,acromioplasty, bursectomy and subacromial decompression of the shoulderjoint; anterior release of the temperomandibular joint; synovectomy,cartilage debridement, chondroplasty, division of intra-articularadhesions, fracture and tendon debridgement as applies to any of thesynovial joints of the body; including thermal shrinkage of jointcapsules as a treatment for recurrent dislocation, subluxation orrepetitive stress injury to any articulated joint of the body;discectomy either in the treatment of disc prolapse or as part of aspinal fusion via a posterior or anterior approach to the cervical,thoracic and lumbar spine or any other fibrous joint for similarpurposes; excision of diseased tissue; and haemostasis.

[0108] The invention is also useful for dissection, resection,vaporisation, desiccation and coagulation of tissue and suchcombinations of these functions with particular application inurological endoscopic (urethroscopy, cystoscopy, ureteroscopy andnephroscopy) and percutaneous surgery. Urological procedures mayinclude: electro-vaporisation of the prostate gland (EVAP) and othervariants of the procedure commonly referred to as transurethralresection of the prostate (TURP) including but not limited tointerstitial ablation of the prostate gland by a percutaneous orperurethral route whether performed for benign or malignant disease;transurethral or percutaneous resection of urinary tract tumours as theymay arise as primary or secondary neoplasms and further as they mayarise anywhere in the urological tract from the calyces of the kidney tothe external urethral meatus; division of strictures as they may ariseas the pelviureteric junction (PUJ), ureter, ureteral orifice, bladderneck or urethra; correction of ureterocoele; shrinkage of bladderdiverticular; cystoplasty procedures as they pertain to corrections ofvoiding dysfunction; thermally induced shrinkage of pelvic floor as acorrective treatment for bladder neck descent; excision of diseasedtissue; and haemostasis.

[0109] The invention is also useful for dissection, resection,vaporisation, desiccation and coagulation of tissue and suchcombinations of these functions with particular application in surgeryon the ear, nose and throat (ENT) and more particularly proceduresperformed on the oropharynx, nasopharynx and sinuses. These proceduresmay be performed through the mouth or nose using speculae or gags orusing endoscopic techniques such as functional endoscopic sinus surgery(FESS). Functional endoscopic sinus procedures may include: removal ofchronically diseased inflamed and hypertrophic mucus linings, polyps andneoplasms from the various anatomical sinuses of the skull; excision ofdiseased tissue; and haemostasis. Procedures on the nasopharynx mayinclude: removal of chronically diseased inflamed and hypertrophic mucuslinings, polyps and neoplasms from the turbinates and nasal passages;submucus resection of the nasal septum; excision of diseased tissue; andhaemostasis. Procedures on the oropharynx may include: removal ofchronically diseased inflamed and hypertrophic tissue, polyps andneoplasms particularly as they occur related to the tonsil, adenoid,epi- and supraglottic region, and salivary glands; as an alternativemethod to the procedure commonly known as laser assisteduvulopalatoplasty (LAUP); excision of diseased tissue; and haemostasis.

[0110] It is evident from the scope of applications of the inventionthat it has further additional applications for dissection, resection.vaporisation, desiccation and coagulation of tissue and suchcombinations of these functions in general laparoscopic, thoracoscopicand neurosurgical procedures being particularly useful in the removal ofdiseased tissue and neoplastic disease whether benign or malignant.

[0111] Surgical procedures using a system incorporating the generator ofthe present invention include introducing the electrode assembly to thesurgical site whether through an artificial (cannula) or naturalconduit, which may be in an anatomical body cavity or space such as thehuman uterus or one created surgically either using the invention oranother technique. The cavity or space may be distended during theprocedure using a fluid or may be naturally held open by anatomicalstructures. The surgical site may be bathed in a continuous flow ofconductive fluid such as saline solution either to fill and distend thecavity or to create a locally irrigated environment around the tip ofthe electrode assembly in a gas filled cavity or on an external bodysurface or other such tissue surfaces exposed during part of a surgicalprocedure. The irrigating fluid may be aspirated from the surgical siteto remove products created by application of the RF energy, tissuedebris or blood. The procedures may include simultaneous viewing of thesite via an endoscope or using indirect visualisation means.

What is claimed is:
 1. An electrosurgical generator for supplying powerto an electrosurgical instrument, the generator comprising a radiofrequency output stage having at least a pair of electrosurgical outputconnections for the delivery of radio frequency power to theinstrurnent, and control circuitry operable to limit the radio frequencypeak output voltage developed across the output connections to at leastfirst and second predetermined threshold values and, in a blend mode ofthe generator, to alternate constantly between said first and secondthreshold values.
 2. A generator according to claim 1, wherein the firstthreshold value is in the range of from 150V to 200V and the secondthreshold value is in the range of from 250V to 600V, the voltages beingpeak voltages.
 3. A generator according to claim 2, wherein the controlcircuity is operable, in said blend mode, to limit said output voltagealternately to said first and second threshold values at an alternationrate in the range of from 5 Hz to 2kHz.
 4. A generator according toclaim 3, wherein the control circuit is operable, in said blend mode, tolimit said output voltage alternately to said first and second thresholdvalues at an alternation rate in the range of from 20 Hz to 1 kHz.
 5. Agenerator according to claim 1, wherein the output stage includes aradio frequency switching device for delivering power signal to theoutput connections, wherein the control circuitry includes sensing meansfor deriving a sensing signal representative of the radio frequency peakoutput voltage developed across said output connections, and a referencesignal generator for generating reference signals representative of saidfirst and second threshold values, wherein the sensing means furtherincludes a comparator arranged to compare said sensing signal with saidreference signals to produce a control signal for actuating theswitching device such as to reduce said delivered output power when thecontrol signal produced by the comparator is indicative of therespective said threshold value having been reached, and wherein thereference signal generator and the comparator are operable, in saidblend mode, to compare said sensing signal alternately with thereference signal representative of the first threshold value and thereference signal representative of the second threshold value.
 6. Agenerator according to claim 5, wherein the reference signal generatorincludes a switching circuit operable to apply said reference signalsalternately to a reference input of said comparator.
 7. A generatoraccording to claim 6, wherein said switching circuit issoftware-controlled.
 8. An electrosurgical system comprising anelectrosurgical generator for generating radio frequency power and anelectrosurgical instrument coupled to the generator, the instrumenthaving an electrode structure for operation immersed in an electricallyconductive liquid, wherein the system has a first mode of operation inwhich tissue is treated by the application of heat in the region of theelectrode structure without forming an electrode-enveloping vapourpocket, and a second mode of operation in which the tissue is locallyvaporised by energy transmitted from the electrode structure via anelectrode-enveloping vapour pocket, said first and second modes beingdefined by different respective electrical control parameters selectedin the generator, and wherein the system has a third, blended mode ofoperation produced by constantly alternating between the first andsecond modes.
 9. A system according to claim 8, wherein the saiddifferent control parameters are first and second radio frequency outputvoltage values, and wherein the generator includes control circuitryoperable, when the blended mode is required, to limit the radiofrequency output voltage of the generator alternately to said first andsecond values.